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Is the line of code that actually toggles the menu. So you could simply add a version of that code above so that it toggles the menu when loaded. Since a toggle is either on or off if it starts on it will start off.
The thing to keep in mind is $(this) is equivalent to $('label.tree-toggler')
So we can adjust the code to:

Now this works but if you are like me you are going to remember the rules of a programmer are don't repeat yourself especially if you are repeating requests to the DOM. So if you find you are going to have to grab this 'label-tree-toggler' multiple times you could use a code such as: